In the same way as a reserve fuel tank in a car enables a car to continue running after it would otherwise have run out of fuel, the term cognitive reserve refers to the brain's ability to keep going despite the degenerative effects of diseases such as Alzheimer's.

The theory is that if a person's brain has more neural connections than another person's, their brain will be more resilient to diseases that destroy neural connections. In other words, if you have more than one connection between two brain cells then your brain has a natural back-up in the event of one connection failing.

We know that new neurological connections are established when we learn new things through a process known as 'neural plasticity'. It should therefore follow that cleverer people and people who keep themselves mentally active should show a greater resistance to dementia, but is there any evidence to support the theory?

The study that is credited with first establishing the link between mental exercise and resistance to dementia was that conducted by Dr Joe Verghese at the Albert Einstein College of Medicine in New York. His study focused on people who were over 75 years of age and looked specifically at the impact their lifestyle choices made on their likelihood of developing dementia.

The study was conducted over a 5-year period and followed the lives of 469 people, none of whom showed any signs of brain degeneration or dementia at the beginning. By the end of the study more than a quarter of the population had developed some form of dementia.

When the data was analysed Dr Verghese found that there was a clear statistical link between the degree to which people exercised their brains through mental activity and the development of dementia. More importantly, it appeared that it was the variety of mentally challenging tasks that were most important is preventing dementia. For example, the people who combined tasks such as reading, playing board games, dancing and playing musical instruments were all associated with a reduced risk.

The study, which was published in the New England Journal of Medicine June 2003, concluded that the participants who did the most activities were 63% less likely to develop dementia as those that did the least and that each additional activity someone did on a weekly basis reduced their risk of developing dementia by a staggering 7%.

A separate study at Western Oregon University suggest that it is never too late to start exercising your brain. Their study looked at how memory is affected by cognitive stimulation. The participants were invited to engage in exercises designed to exercise different parts of their brains 3 time a week for just 3 months. What they found was that the actual memory ability of the participants improved by around 7%.

However, while both of these studies support the hypothesis that the brain is a 'use it or lose it' organ (a term first coined in relation to the brain by Dr Verghese), they do not specifically make the link between intelligence and dementia.

A link does however appear to be implied by research in Australia where a 10-year study of 26,000 people by Dr Michael Valenzuela of the University of New South Wales found that people with a history of complex brain activity developed greater cognitive reserve and were 46% less likely to develop dementia.

The problem with all of this research is that it does not explicitly separate the effects of recent mental exercise from historic learning and so does not conclusively prove that intelligence alone is a factor influencing dementia resistance.

Conclusion

Since learning causes brain cells to create additional links to one another it is logical to assume that there is a link between cognitive reserve and intelligence.

There is also evidence to suggest that the greater a person's cognitive reserve, the greater their resilience to degenerative conditions such as Alzheimer's.

But as yet there does not appear to be any conclusive evidence to suggest that an intelligent person is any less likely to develop a neurologically degenerative condition than a less intelligent person who regularly engages in a variety of mentally challenging tasks. There is however plenty of evidence to suggest that people who are more intelligent or who have had a better academic education in early life are more likely to engage in a wider variety of mentally challenging activities throughout their lives.

The only real conclusion we can draw therefore is that the real enemy of mental vitality is not age, but boredom.